1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or printer, and particularly to an image forming apparatus and image forming method in which latent images on an image bearing body are made visible with a developer, and a process cartridge used therefor. The invention also relates to an image forming method using a one-component magnetic toner.
2. Description of the Related Art
A number of electrophotographic processes are known in the art. In the electrophotographic process, a fixed image is formed through a plurality of steps comprising electrically forming a latent image by various methods on an electrostatic latent image bearing body utilizing a photoconductive material, developing the latent image using a developer containing a toner, transferring the toner latent image on the electrostatic latent image bearing body onto a transfer body such as a sheet of paper with or without interposition of an intermediate transfer body, and fixing the latent image by heating, compressing or heating with compression, or with a vapor of a solvent. If necessary, toner remaining on the latent image bearing body is cleaned off by various methods, and the aforementioned plural steps are repeated. Printers and copying machines utilizing the electrophotographic process are widely used, and accordingly, requirements with respect to performance and image quality are become more strict year by year.
Developing methods for the electrophotographic process are divided into one-component developing methods and two-component developing methods. While the two-component developing method has widely been used for development since it is advantageous for high speed processing, it involves disadvantages such as deterioration of the developer due to adhesion of the toner on the surface of a carrier and a large size of the developing device since the mixing ratio of the toner to the carrier should be kept constant so that the toner concentration in the developer does not decrease due to only the toner being consumed in this method. Consequently, the cost of controlling the toner density becomes high. On the other hand, since the one-component developing method is advantageous in that the device is compact and the cost may be reduced without producing the above-mentioned defects, the device is prevalent in small offices and in the field of personal users.
The one-component developing method is roughly divided into a non-magnetic one-component developing method and magnetic one-component developing method. The former is suitable for color printing because the toner does not contain magnetic powders. On the other hand, the magnetic one-component developing method is frequently used in a monochromatic electrostatic copying method since the toner can be retained on the toner bearing body using a magnetic force of the magnetic powder contained in the toner, and from the viewpoints of good conveying ability of the toner and easily inhibition of fogging of the toner at non-image portions.
The one-component developing method is more suitable for making the apparatus compact as compared with the two-component developing method. However, while so-called long life is urgently required for increasing the number of printing sheets (the number of copying sheets) per process cartridge in addition to the requirement for more compactness in recent years, a space for accommodating the developer is further reduced by making the process cartridge small. Consequently, the number of printing or copying sheets printed or copied before the toner in the cartridge has been depleted is reduced. Therefore, various methods have been devised for satisfying requirements of both small size and long life.
For example, a compact apparatus has been realized by disposing the developing section, exposing section and cleaning section at the same side relative to a vertical face intersecting the center of rotation of the image bearing body in the process cartridge used for the image forming apparatus disclosed in Japanese Patent Application Publication (JP-B) No. 6-12475. Further, Japanese Patent Application Laid-Open (JP-A) No. 2002-040787 discloses a method in which the developing device is made compact by simplifying the construction thereof in order to ensure a sufficient space for accommodating the developer. However, there has been a problem in that undeveloped developer remains in a dead space in the cartridge.
In order to reduce the size, and particularly the width, of the apparatus, it has been proposed to arrange a recording medium path from a transferring step to a fixing step in a substantially vertical direction to discharge the recording medium at the upper portion of the apparatus instead of discharging the recording medium to the side of the apparatus. However, since a developing device is positioned below a scanning light path in a process cartridge image forming apparatus such as described above, the size of the developing device must be small for reducing the height of the apparatus. Since the developing device is equipped with a developer storage portion for storing the developer, the size of the developer storage portion is inevitably small, whereby it is difficult to reduce the size of the apparatus with the volume of the developer storage portion unchanged.
It is also a drawback of the one-component developing method that toner conveyability onto the toner bearing body is likely to be unstable and distribution of electric charges of the toner tends to be broadened, since a charge donating ability, a developer conveying ability, and a developer stirring ability of the carrier cannot be utilized in this method. Consequently, development ghosts, fogging of the toner at non-image portions, and a decrease in development density are liable to occur. Since a darker black hue is required than conventionally from the viewpoint of improving image quality, higher image density is demanded. On the other hand, the foregoing problems are more liable to occur due to making members such as the toner bearing body compact in accordance with the trend toward a small-sized apparatus, due to shortening the charging time and developing time for high speed processing, and due to reducing the charge amount per toner particle in accordance with a reduced particle size of the toner for obtaining high quality images.
A variety of proposals have been made to solve the aforementioned problems.
It has been found that a development ghost could usually be removed using a method for suppressing charge-up of the toner. In a technology utilizing the above-described discovery, electrical resistance of the developer holding member (sleeve) is reduced to lower the absolute value of the toner charge by providing, for example, a resin layer comprising a phenol resin and carbon and having electrical conductivity and surface lubricity, on the toner bearing body in order to prevent a development ghost from occurring (e.g., see JP-A No. 1-276174). However, developing ability from the developer bearing body to a latent image bearing body (photoreceptor) may be deteriorated by reducing the charge amount of the toner, resulting in a decrease in image density. Since the charge amount per toner particle has been reduced in recent years by forming toner with a smaller particle diameter for complying with the requirement of high image quality, fogging is liable to occur. In another method disclosed, for example, in JP-A No. 10-177275, a developer bearing body having on its surface a film that contains Mo, O and H as main components is used, and a titanium compound obtained by a reaction between TiO(OH)2 and a silane compound is added to a magnetic one-component developer. A layer of toner transferred onto the bearing body is made uniform using the developer bearing body having the Mo-based material, and a ghost is prevented from being generated by utilizing a rapid charge exchanging property by adding the titanium-based compound. This method is quite effective. However, in a contact type transferring system, an electric field may affect the toner since the titanium-based compound as an additive has a relatively low electrical resistance to thereby reduce transferring efficiency and image density. These phenomena are apt to occur in a high temperature/high humidity environment and in a low speed machine having a long transferring time. Transferring ability is also liable to be impaired due to a decrease of the charge amount resulting from reduction of the particle diameter as described above. Further, the amount of the residual toner on the latent image bearing body is certain to increase, even when impaired transferring ability causes no problem with respect to image quality. Thus, further improved transferring efficiency is required from the viewpoint of reducing the amount of wasted toner.
With respect to deterioration of the transferring ability due to the electric field, a technique has been proposed in which the transferring ability may be improved at a process speed below 120 mm/s by optimizing a change in the resistance of the toner (e.g., see JP-A No. 2002-169329). However, in such a one-component magnetic toner, when the process speed exceeds 120 mm/s, worsening of a development ghost and a decrease in image density, due to high resistance of the toner as a result of speeding-up of the process, becomes more prominent than the transferring ability improving effect. Further, when the development electric field is increased to obtain higher image density, an increase in the process speed is accompanied by an increase in toner that contributes to fogging. Therefore, compatibility of transferability and developing ability is a current issue.
As a toner capable of complying with high speed processing, a toner has been proposed, which uses a magnetic powder whose magnetic property, fluidity, environmental stability and resistance can be adjusted by a composite oxide film of aluminum and iron, and which does not reduce the blackness of the toner (see, for example, JP-A No. 2002-72545). When the magnetic powder is used in the magnetic toner, excellent developing ability and transferability can be achieved by decreasing the content of the magnetic powder in the toner in a low process speed region below 120 mm/s. On the other hand, the toner can effectively be used when the process speed exceeds 120 mm/s by increasing a content of the magnetic powder depending on the process speed and using an additive having a relatively low resistance such as a titanium-based additive. However, even if such a means is adopted, the content of the magnetic powder must be adjusted depending on the process speed, and currently available one-component magnetic toners cannot satisfy all of the properties such as developing ability and transferability in a region of from a low speed (120 mm/s or less) to a high speed (up to 250 mm/s) when one kind of toner is used.
Moreover, the fixing ability of the toner is not sufficient when the process speed is 180 mm/s or more and a heating time for fixing is shortened. From this standpoint as well, conventionally known one-component magnetic toners cannot exhibit sufficient ability in the region of from a low speed (120 mm/s or less) to a high speed (up to 250 mm/s) when one kind of toner is used.